From the report:
Macs differ drastically from Windows in terms of the types of threats seen. Where we found several different categories and families in our top detections of Windows threats that classify as traditional malware, especially those aimed at businesses, most Mac threats, and certainly the most prevalent ones of 2019, are families of adware and potentially unwanted programs (PUPs). The most common Mac malware family, OSX.Generic.Suspicious, fell well down the list at 30th place in Mac-specific detections, and hundreds of spots down on a cross-platform threat list.
Of all the [Mac] threats seen this year, only one incident involved anything other than tricking the user into downloading and opening something they shouldn’t. That is the incident in which Coinbase, and several other cryptocurrency companies, were targeted with malware that infected systems through a Firefox zero-day vulnerability.
So the chance of experiencing malware — not adware or what Malwarebytes calls “potentially unwanted programs”, but malware — on a Mac actually fell in 2019, according to this report. Meanwhile, as Ben Lovejoy points out, the primary reason adware became more prevalent on the Mac in 2019 is down to a single app.